Possible side-story/in-universe legend thing: Jeanne d'Arc and her blade Almace, a dark copy of the lost Durendal. Things went a bit differently for Inm!Jeanne than her historical basis.

Further elaboration: my original idea was a straight-up Jeanne/Roland-or-other-paladin team up, which was squashed when I checked the dates. Revision # 1 was "Jeanne being temporarily entrusted with Durendal", but then that would've hosed up the idea of the Eternal Hero being the only one to wield that sword. Revision # 2 was remembering that I'd once drawn Almace-the-sword (as opposed to Almace the alternate-timeline fusion of Durandal and Tycho) and saying that it had some relevance to Inmortalitas...

Almace-the-sword was an attempt to recreate the by-then legendary Durendal that...didn't quite go right, violently rejecting all of its potential wielders except for Jeanne d'Arc. Despite the risks involved in using such a dark blade, Jeanne took Almace for herself, believing that it could help her turn the tide of the Lancastrian War. It proved to be useful in dealing with the less-human threats that emerged soon after...